The present invention relates to a memory circuit, and more particularly to a memory circuit having a function of serially reading from or writing to a plurality of memory cells.
A memory circuit called a serial memory comprises a memory cell array having a plurality of memory cells arranged in a matrix form of rows and columns and a shift register which serially addresses a plurality of memory cells in the designated row at a high speed. In a serial memory utilized in video field, information obtained by serially reading memory cells in each row are used for the display in one line on a screen of a CRT, or other display means. In the typical structure of the serial memory which employs a memory array of M rows and N columns, a row decoder selects one of M rows and data of the N memory cells in the selected row are sequentially read out or written through N columns by serially selecting the N columns by a shift register of N stages. In the shift register, "1" level output is shifted from the initial stage of the N stages towards the last, Nth stage one by one in synchronism with shift clocks.
In the above-mentioned serial memory, the shift register is required to have N stages for selection of N columns of memory cells. In general, each stage of the shift register includes at least six transistors, and therefore, the number of transistors in total required for the shift register of N stages is very large. Therefore, it is difficult to fabricate the shift register on a small space of the semiconductor chip. Particularly, it is difficult to arrange each stage of the shift register with a pitch of the columns of the memory array. Thus, the shift register is an obstacle for a high density serial memory.
Moreover, in case where the number of N is large or the stages of the shift register is large, the length of feed-back path from the last stage to the initial stage becomes large and therefore a large capacitance is inevitably associated therewith. Thus, there is a possibility that the last stage and the initial stage are both selected, resulting in mulfunction.